Both the medical and nonmedical use of opiate drugs have greatly increased in the U.S. in recent years. This increase is especially noteworthy in the increased number of teenagers abusing prescription opiate drugs. Despite this increase, very little is known about the neuroadaptations that occur with chronic opiate use. The goal of this project is to identify the underlying molecular signals that contribute to neuroadaptation to chronic opiate use, specifically the mechanisms that cause a decrease in the size of dopaminergic ventral tegmental (VTA) neurons and the corresponding behavioral adaptation of reward tolerance. Previous work in the Nestler laboratory has established a chronic morphine paradigm that decreases the size of dopaminergic cells in the VTA in a BDNF- and PI3K/Akt-dependent manner. Further, changes in PI3K/Akt signaling in the VTA were found to be necessary and sufficient to produce reward tolerance. This project aims to determine what signaling pathways downstream of Akt modulate this change in cell size and behavior. One prominent signaling pathway that is known to affect cell size in the CNS as well as in the periphery is the mammalian target of rapamycin (mTOR) pathway. We hypothesize that alterations in mTOR signaling contribute to morphological and behavioral changes produced by chronic morphine. To determine whether mTOR signaling is altered in response to chronic morphine, rats will be treated with morphine and the VTA will be dissected for analysis of biochemical changes by western blot. Additionally, the mTOR inhibitor, rapamycin, will be used to inhibit mTOR signaling in order to determine whether decreased mTOR signaling can block morphine-induced reward tolerance. Finally, local viral-mediated overexpression of wildtype and activating and dominant-negative forms of a key modulator of mTOR signaling, ras homolog enriched in brain (Rheb) will be used to mimic changes induced by chronic opiates and to block chronic opiate effects. Given that the most highly prescribed drug in the U.S. in 2005 was an opiate, hydrocodone with acetaminophen, and that the non-prescription use has also increased, especially in adolescents where use of the opiates Oxycontin and Vicodin is greater than more studied drugs of abuse such as cocaine, investigating the action of opiate drugs on neurobiology and behavior is an important and topical drug abuse project. The goal of our work is to understand the neuroadaptations that occur with chronic opiate use, which may help us to better understand both the addiction liability as well as the phenomenon of reward tolerance, decreased reward for drug leading to escalation of dose.